1. Field of the Invention
The invention relates to a hardening method of an annular workpiece made of metal.
2. Description of Related Art
A bearing ring mainly made of steel of a rolling bearing, for example, as an annular member uses steel for a bearing, such as bearing steel or carburized steel. In order to give the bearing ring the desired mechanical strength, heat treatment such as hardening (quenching) must be applied to the annular workpiece. When the annular workpiece is hardened, the roundness deteriorates, and dimensional variation of the outer diameter and the inner diameter increases, which is problematic.
As a method for suppressing variation of the outer diameter and inner diameter of the annular workpiece, Japanese Patent Application Publication No. 2014-62308 (JP 2014-62308 A), for example, proposes a method that involves performing hardening treatment using a hardening device that includes an outer periphery restraining device that restricts deformation of the annular member toward the radial outside by abutting against an outer peripheral surface of the annular member, and an inner periphery restraining device that restricts deformation of the annular member toward the radial inside by abutting against an inner peripheral surface of the annular member